1. Field of the Invention
The present invention relates to an electrostatic ejection type ink jet head that controls ejection of ink by means of an electrostatic force.
2. Description of the Prior Art
In an electrostatic ejection type ink jet recording system, ink containing a charged fine particle component is used and a predetermined voltage is applied to each individual electrode of an ink jet head in accordance with image data, thereby controlling ejection of the ink by means of an electrostatic force and recording an image corresponding to the image data on a recording medium. As a recording apparatus adopting this electrostatic ejection type ink jet recording system, an ink jet recording apparatus disclosed in JP 10-230608 A is known, for instance.
FIG. 21 is an example of a conceptual diagram showing a schematic construction of an ink jet head of the ink jet recording apparatus disclosed in the above patent document. In this drawing, an ink jet head 350 is shown as the ink jet head of the disclosed ink jet head recording apparatus, with only one of individual electrodes constituting the ink jet head being conceptually illustrated. Also, the ink jet head 350 includes a head substrate 312, an ink guide 314, an insulating substrate 316, a drive electrode 352, and a counter electrode 322.
Here, the ink guide 314 is arranged on the head substrate 312, and a slit serving as an ink guide groove 326 is formed in the center portion of the ink guide 314 in the top-bottom direction on the paper plane of this drawing. Also, in the insulating substrate 316, a through hole 328 is established at a position corresponding to an arrangement of the ink guide 314. The ink guide 314 is allowed to pass through the through hole 328 established in the insulating substrate 316 so that the tip portion thereof protrudes above the upper surface of the insulating substrate 316 in the drawing.
Also, the drive electrode 352 has a ring shape and is provided for each individual electrode on the upper surface of the insulating substrate 316 in the drawing so as to surround the periphery of the through hole 328 established in the insulating substrate 316. Further, the head substrate 312 and the insulating substrate 316 are arranged with a predetermined space therebetween, and an ink flow path 330 is formed between the substrates 312 and 316. Also, the counter electrode 322 is arranged at a position opposing the tip portion of the ink guide 314 and a recording medium P is placed on the lower surface of the counter electrode 322 in the drawing.
Also, FIG. 22 is an example of a conceptual construction diagram of a drive circuit for the drive electrode.
The drive circuit 354 in this drawing includes an FET (field-effect transistor) 334 and resistive elements 336 and 338. A drain of the FET 334 is connected to the drive electrode 352, a source of it is connected to ground level, and a gate of it receives input of a control signal. Also, the resistive element 336 is connected between a high-voltage power supply and the drive electrode 352, while the resistive element 338 is connected between the control signal and the ground level.
In the drive circuit 354, the control signal is changed between high level and low level in accordance with image data. When the control signal is set to the high level, the FET 334 is turned on and the drive electrode 352 becomes the ground level. On the other hand, when the control signal is set to the low level, the FET 334 is turned off and the drive electrode 352 becomes the high-voltage level of the high-voltage power supply. That is, the drive electrode 352 is frequently switched between the ground level and the high-voltage level in accordance with the image data.
At the time of recording, ink containing a fine particle component and charged to the same polarity as the high-voltage level applied to the drive electrode 352 is circulated in a direction from the right to the left in FIG. 18.
When the drive electrode 352 is set as the ground level, the electric field strength in proximity to the tip portion of the ink guide 314 is reduced, and therefore, the ink will not fly out from the tip portion of the ink guide 314. In that case, a part of the ink moves upward along the ink guide groove 326 formed in the ink guide 314 due to capillary action until above the upper surface of the insulating substrate 316 in the drawing.
On the other hand, when the high-voltage level is applied to the drive electrode 352, the ink that moved upward along the ink guide groove 326 of the ink guide 314 until above the upper surface of the insulating substrate 316 in the drawing flies out from the tip portion of the ink guide 314 due to a repulsion force. The ink is then attracted to the counter electrode 322 biased to a negative voltage level and adheres onto the recording medium P.
The ink jet head 350 and the recording medium P placed on the counter electrode 322 are relatively moved during this operation, thereby recording an image corresponding to the image data on the recording medium P.
By the way, when a recording apparatus is required to perform high-definition recording at high speed, a line head that is capable of recording one line of an image at a time inevitably becomes necessary. When the definition and recording speed of the recording apparatus are respectively 1200 dpi (dot/inch) and 60 ppm (page/minute), for instance, a line head that is capable of recording an image on a recording medium having a width of 10 inch needs to include many individual electrodes, whose number is 12000 that is equal to the number of pixels on one line, and drive circuits whose number is equal to the number of the individual electrodes to be driven.
In this case, the individual electrodes and the drive circuits need to be implemented in the line head at a physically extremely high density with reference to the line direction. The drive circuits use high voltage (around 600 V, for instance), so that when the individual electrodes and the drive circuits are arranged at a high density, a danger of discharge is increased. Accordingly, it is extremely difficult to cope with both high-density implementation and high-voltage operation.
Also, in the drive circuits described above, if it is assumed that current of 1 mA flows to each individual electrode, the total current flowing to the 12000 individual electrodes becomes up to 12 A. Accordingly, when switching to high voltage of 600 V is performed, the power consumption becomes 7.2 kW. Even if an efficiency of the high-voltage power supply is assumed 100%, a power source of AC 200 V and 36 A is required. Even in that case, only the recording of a monochrome image on an A4-size recording medium is possible, which means that such a system is too much unrealistic.
When a FET is used to perform the switching like the drive circuit described above, it is principally required to flow a certain current to the FET in order to maintain switching speed. In contrast to this, the drive electrode is so minute ring-shaped electrode that the amount of a current consumed by ink ejection itself is around 50 nA at most and is extremely small. That is, most of the current supplied from the high-voltage power supply is consumed by the switching of the FET.